/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode* head) {
        if (head == nullptr) return false;
        ListNode* left = head;
        ListNode* right = head;
        while (right->next && right->next->next)
        {
            left = left->next;
            right = right->next->next;
            if (left == right)
            {
                return true;
            }
        }
        return false;
    }
};